Up to now, we have assumed that the number of particles contained
in a given system is a fixed
number. This is a reasonable assumption if the particles possess non-zero
mass, since we are not generally considering
relativistic systems in this course.
However, this assumption breaks down for the case of photons, which are
zero-mass bosons. In fact, photons enclosed in a container
of volume , maintained at temperature , can readily be absorbed
or emitted by the walls. Thus, for the special case of a gas of
photons there is no requirement which limits the total number of particles.

It follows, from the above discussion, that photons obey a simplified
form of Bose-Einstein statistics in which there is an unspecified
total number of particles. This type of statistics is called photon
statistics.

Consider the expression (584). For the case of photons, the
numbers
assume all values
for each ,
without any further restriction. It follows that the sums in the
numerator and denominator are identical and, therefore, cancel. Hence,
Eq. (584) reduces to

(598)

However, the above expression can be rewritten

(599)

Now, the sum on the right-hand side of the above equation is an infinite
geometric series, which can easily be evaluated. In fact,